Electrochemical fluorination is a process for production of perfluorinated inorganic and organic compounds that has been known for a long time. Its particular advantage, according to U.S. Pat. No. 2,519,983, is the possibility of fluorinating organic compounds while preserving functional groups, as for example a sulphonyl fluoride group.
Perfluoroalkylsulphonyl fluorides, particularly perfluorooctylsulphonyl fluoride, are used for production of perfluorinated organic compounds that are used for example as active agents in textile and leather finishing, in fire-extinguishing agents and in electroplating technology.
Electrolytic fluorination has been described in a large number of publications. A summary can be found for example in the journal Chem.-Ing.-Tech. 58, 31-38 (1986). Electrofluorination is usually carried out in anhydrous hydrogen fluoride at nickel electrodes with cell voltages of 4 to 6 V. The important disadvantage of the procedures described is that perfluoroalkylsulphonyl fluorides, especially those with more than 5 carbon atoms, can be obtained in fairly prolonged continuous production only in low space-time yield. In this connection the space-time yield specifies how much of the desired product can be produced per unit of time and of anode geometrical area.
To improve the space-time yield it is proposed in U.S. Pat. No. 3,919,057 to carry out the electrolysis at cell voltages of 9 to 15 V. But in continuous, prolonged operation these high cell voltages can lead even after a few days or weeks to falling material yields of the desired perfluoroalkylsulphonyl fluorides. In addition such high cell voltages lead to a large accumulation at the electrodes of sludge caused by by-products as well as, within a short time, to the destruction of the electrodes, so that economic production of perfluoroalkylsulphonyl fluorides, especially those with more than 5 C atoms in the alkyl group, is not possible.
It was therefore the problem of the present invention to make available a process that does not have the aforementioned disadvantages, so that economic production of perfluoroalkylsulphonyl fluorides is possible.